1. Field of the Invention
The present invention relates to a super-quick freezing method and apparatus capable of preserving the freshness of food ingredients and food products and enabling long-term storage thereof, and further capable preservation and storage of living cells.
2. Description of the Related Art
Various freezing methods and freezing apparatuses have been developed as a means of storing food ingredients and food products while preserving its freshness for a long period of time. However, when freezing perishable foods such as fishery products, it was not possible to fully prevent (1) emission of offensive odor after freezing and defrosting, (2) discoloration, (3) deterioration in taste, and (4) dripping (effusion of juice upon defrosting). (1) to (3) result from putrefaction of the food ingredients caused by bacteria which are increased in number and oxidization of the food ingredients. Dripping, as in (4), occurs because of the long period of time required for freezing. That is, ice crystals, which are formed by freezing free water existing in an object-to-be-frozen such as food ingredients and food products, become too large and bulky, and cause damage in cellular structure. (Free water is water which is not under constraint of proteins or such and which can move freely.) In other words, because it takes such a long time to go through a temperature range of 0.degree. C. to -20.degree. C., which is the temperature range in which solidification begins and terminates, the ice crystals become too large and bulky, causing the above dripping.
In order to prevent these problems, a freezing technique, as disclosed in Japanese Patent Laid-open Publication No. 10-179105, has been proposed recently.
In the above-proposed freezing technique, freezing is carried out by directly immersing an object-to-be-frozen into a liquid coolant, or by spraying a liquid coolant to the object, in order to accelerate the cooling rate. This restrains the ice crystals from growing during freezing, and prevents destruction of cellular structure. Further, by applying an electromagnetic energy to the object-to-be-frozen, the size of water clusters is reduced. (These will be referred to hereinafter as "small clusters".) These small clusters can easily penetrate into the object-to-be-frozen. Because of the effect of restraining bacteria from increasing, which effect the small clusters possess, the number of live bacteria can be limited, resulting in improvement of quality.
However, in the above-proposed method, since methanol, ethanol, acetone or like materials are used as the liquid coolants, in many cases it is not possible to directly immerse the object-to-be-frozen thereinto. In order to avoid direct contact of the liquid coolant with the object-to-be-frozen, it is necessary to provide a pre-freezing process for packing the object into a container or like packages. When freezing by direct immersion, it will also be necessary to provide a post-treatment process of cleaning/removing the liquid coolant off from the surface of the object after defrosting. The above processes are inconvenient, and will take some amount of time, during which deterioration in freshness may occur.
Furthermore, in the above freezing method, the cooling and freezing is brought about by heat transmission caused by the contact with the liquid coolant, and thus, the cold is transmitted from the outer surface towards the inside. Therefore, freezing starts from the outer surface of the object-to-be-frozen, and proceeds gradually towards the inside. In other words, a frozen layer is formed firstly on the outer surface, and freezing progresses towards the inside. During this process, the cold goes through the initially-formed outer frozen layer, and is then transmitted towards the inside, so the transmission is greatly inhibited by the frozen layer. Thus, it takes a considerably long time for the freezing to complete, especially at the inner core, making it difficult to fully prevent destruction of cellular structure.
Further, when using freezing-storage in the field of biomedical transplant, destruction of cellular structure and extension in operation time becomes fatal, and thus, the above proposed freezing method cannot be adopted.